99 suburban 5.7 vortec stumble just off idle

This is a discussion on 99 suburban 5.7 vortec stumble just off idle within the Technical / Maintenance forums, part of the General Discussion category; mornin all,
As the title says, 99 suburban 5.7 vortec engine.
slightly hard to start, idles smooth, but has a ...

99 suburban 5.7 vortec stumble just off idle

slightly hard to start, idles smooth, but has a severe stumble as soon as you dip in to the throttle. if you feather the throttle, you do not experience it, you have to get 15-20% into it to make it stumble.
This only happens at idle, not at any other point, so the only time you see it is at a stop.

I have removed the throttle body, and cleaned it. The IAC got a good cleaning as well, and since the TPS was giving suspect readings (dropped out in spots) it got replaced, and the new one has been checked, with good results.
I have also been checking for vac leaks, and am unable to find any.

Re: 99 suburban 5.7 vortec stumble just off idle

Rule #1 when troubleshooting an EFI engine, check the fuel pressure. Easy to do on the Vortec - small block has the test fitting right next to the black plastic square "Vortec" box. On the small block you want to see a minimum of 58psi. Let us know whatcha get.

Re: 99 suburban 5.7 vortec stumble just off idle

I am on pump #3 this time with an OEM pump. kind of aware of the normal issues with fuel pressure, this seems different since once past the stumble, it pulls very strong at any throttle setting all the way to redline.

that being said, I will check it.

Rob

ps, just ran it up to a shop, he dropped the scanner on it (free of charge), and no codes were showing. but I was able to demonstrate it to him, he suspects the cap/rotor/plug wires.

Re: 99 suburban 5.7 vortec stumble just off idle

Originally Posted by helowrench

I am on pump #3 this time with an OEM pump. kind of aware of the normal issues with fuel pressure, this seems different since once past the stumble, it pulls very strong at any throttle setting all the way to redline.

that being said, I will check it.

Rob

ps, just ran it up to a shop, he dropped the scanner on it (free of charge), and no codes were showing. but I was able to demonstrate it to him, he suspects the cap/rotor/plug wires.

I am having the same problem in my 97 'burban, it won't do it all the time but I get the stumble. I replaced the cap,rotor,plugs & wires and it is still doing it.

Re: 99 suburban 5.7 vortec stumble just off idle

Also check for pressure bleed off after the pump turns off. It should hold pressure overnight, at least for several hours. Bleed off can point to a bad check valve in the pump or a leaking pressure regulator. Of course make sure your gauge isn't bleeding any pressure.

Re: 99 suburban 5.7 vortec stumble just off idle

Originally Posted by 99-2door

Also check for pressure bleed off after the pump turns off. It should hold pressure overnight, at least for several hours. Bleed off can point to a bad check valve in the pump or a leaking pressure regulator. Of course make sure your gauge isn't bleeding any pressure.

my gauge is a european car unit for Bosch K-Jet, which gives me a positive shut off valve, and much higher pressure reading capabilities.

Re: 99 suburban 5.7 vortec stumble just off idle

I have got the stumble going on as well. Never in problems starting or anything just a stumble and lower RPM's if I dump the throttle. I recently replace the spark plugs, cable, fuel filter, and fuel pump. Still got it. Mine is 1999 2door tahoe. Man is it frustrating. Any one have suggestions beside what has been said in this thread. I will try and check the fuel pressure soon. Truck drives great otherwise just this stumble.